1. Field of the Invention
The present invention relates to a free cutting graphitic steel in which spheroidized graphite is contained in an amount between 0.20 and 0.90% and distributed at a ratio of more than 50 graphites (as defined herein) per square millimeters, so as to assure the desired machinability, and the C and Si contents are controlled so as to improve the hot workability.
2. Description of the Prior Art
The elements used in a free cutting steel to impart machinability to the steel, are primarily, S, P, Se, Pb, and Te. Among the various groups of free cutting steels, those steels in which sulfur is present and MnS is utilized as chip breaker are the most commonly used. However, the sulfur steels suffer the disadvantage that the properties are largely dependent upon the steel direction due to the development of banded structures and deterioration of the mechanical properties and hot shortness during hot working.
Meanwhile, Pb is used mostly in free cutting steels which are required to have good material quality since Pb remarkably improves the machinability without substantially effecting the mechanical properties.
At the present time, therefore, the free cutting steels which are commercially available may be classified largely into the sulfur steel group, the leaded steel group and the lead or sulfur complex steel group, and elements other than sulfur and lead are used only for an auxiliary role in the free cutting steels.
Lead is effective to increase the notch effect of a steel by adhering to the sulfides and oxides in the steel or by its distribution alone in the steel. It is effective to impart lubricity to the tool surface by its dissolution caused by the temperature rise during the machining operation.
Therefore, lead is more advantageous than sulfur from the standpoint of tool life improvement. However, since the air pollution problem has been of great concern in recent years, increased cost and labor are required for dust removal and handling during production and machining of leaded free cutting steels. Thus, strong demands have been made for free cutting steels which can be substituted for the lead-containing free cutting steels.
Steels utilizing graphite, e.g., a graphitic steel and a free cutting carbon tool steel are conventionally known. However, in these conventional steels, graphite is not utilized directly for improving the machinability of a structural steel.
For example, in the conventional graphitic steel, part of the carbides is decomposed into free graphite, but the combined carbon content is maintened at not less than 0.7%, so as to assure the desired heat treatment properties, and the steel has been used limitedly for parts, such as, drawing dies, taps, rolls and spindles which require good wear resistance.
Also, in the conventional free cutting carbon tool steel, graphitization is suppressed to 0.1 to 0.4% at the highest because excessive graphitization of the carbide lowers remarkably the heat treatment properties, and the machinability is maintained by the addition of Pb, S, Te, Se, etc.
In any event, both of the above conventional steels have been directed to wear resistant steel parts and tools, and are not directed to parts which are produced in mass by automatic machine tools. Yet, both of the conventional steels have poor hot workability properties and cannot be produced economically on a commercial scale by hot rolling with a rolling mill.